Electron energy filter

ABSTRACT

An electron energy filter includes a first pair of magnetic poles for generating a first deflecting magnetic field and a second pair of magnetic poles for generating a second deflecting magnetic field in the same direction as the first deflecting magnetic field. The incident electrons are deflected about 90° with a trace radius of AM1 through the effect of the first deflecting magnetic field, passed through a free space having a distance DL2 that is about a half of the trace radius AM1 and then are incident to the second deflecting magnetic field. The electrons are deflected about 180° with a trace radius AM2 that is about a half of the curvature radius AM1 and are passed through the free space DL2. Then, the electrons are incident to the first deflecting magnetic field again where those electrons are deflected about 90°. The deflected electrons are traveled like a gamma trace so that those electrons outgo in the same direction as the incident one. This filter so designed is made compact and to have a smaller aberration.

TECHNICAL FIELD

The present invention relates to an electronic energy filter which isarranged to separate only electrons having specific energy from anelectron beam and form an image of those electrons.

BACKGROUND ART

In a transmission electron microscope, electrons transmitted through aspecimen suffer from energy loss that is peculiar to one or moreelements composing the specimen. To overcome this shortcoming, theelectrons transmitted through the specimen are passed through an energyfilter for analyzing energy of those electrons, separating only theelectrons suffering from the specific energy loss, and forming an imageof the separated electrons. The formed image corresponds to a mappingimage of one or more specific elements contained in the specimen.Further, the use of only the electrons having specific energy forforming an image allows the energy loss of the electrons caused by thethickness of the specimen to be restricted to only specific electrons.The resulting image has excellent contrast.

For the electron energy filter used for this kind of purpose, there havebeen known an omega type energy filter (U.S. Pat. No. 4,740,704) and analpha type energy filter (U.S. Pat. No. 4,760,261).

The omega type energy filter is composed of three electromagnets asshown in FIG. 2. The first electromagnet 1 has an opposite deflectingdirection to the second and the third electromagnets 2 and 3. Incidentelectrons 4 are traced like an omega (Ω) and are fired in the samedirection as the incident one, for selecting only the electrons havingspecific energy loss. As shown, a numeral 5 denotes a crossover point. Anumeral 6 denotes an inlet image surface. A numeral 7 denotes an outletimage surface. A numeral 8 denotes an energy dispersion surface.

The alpha type energy filter is composed of three electromagnets 11, 12and 13 having the same deflecting directions as each other as shown inFIG. 3. Incident electrons 4 are traced like an alpha (α) and finallyoutgoes in the same direction as the incident one, for selecting onlythe electrons that suffer from specific energy loss. Another kind ofalpha type energy filter is shown in FIG. 4 (Perez, J. P., Sirven, J.,Sequela, A., and Lacaze, J. C., Journal de Physique, (Paris), 45, Coll.Cs, 171 to 174 (1984)). This energy filter is constructed so that anelectromagnet 14 having a deflecting angle of 70° is located as opposedto the other electromagnet 15 having a deflecting angle of 220° withnarrow middle space 16, in which those electromagnets yield thecorresponding magnetic field intensities. In the construction, anincident electron beam 4 goes around the inside of the filter foranalyzing the energy of the electron beam.

DISCLOSURE OF INVENTION

The electron energy filter is required to form an energy dispersionsurface and an image surface with a small aberration. That is, theelectrons 4 transmitted through the specimen enter at the crossoverpoint 5 formed on a lens into the electron energy filter, pass throughthe filter and reach the energy dispersion surface 8 where the electronshaving the specific energy are converged in the same direction thoughthe electrons having the other energy are dispersed. On the energydispersion surface 8, hence, the aberration has to be eliminated inorder to restrict a lower resolution of the electrons passing through aslit. Moreover, the image formed on an inlet image surface 6 before theelectron energy filter is required to be similarly formed on an outletimage surface point 7. Hence, the aberration has to be reduced to aminimum for suppressing the distortion of the image as much as possible.

A convergence condition of the electron optical system provided in theelectron energy filter arranged to use a fanlike electromagnet may becalculated by using a calculation program of an ion optical system usedfor designing a mass spectrometer. The convergence characteristic up toa three degrees, which considers the effect of an end magnetic field,can be precisely calculated by the calculation program TRIO that iscompleted by Matuo, Matuda, et al. (T. Matsuo, H. Matusda, Y. Fujita andH. Wollnik; Mass Spectroscopy, Vol. 24, No.1, March 1976)

The electron optical system of the aforementioned conventional electronenergy filter does not make sufficient allowance for the effect of theend magnetic field. The image point formed after the passage of theelectrons through the electron energy filter is required to make theenergy dispersion disappear, converge the image in two directions, andhave the magnifications of the images kept at a value of 1. As mentionedbelow, the electron optical system does not completely meet theseconditions.

Assuming that the energy dispersing direction is x and the directioncrossed at right angles with the direction x (direction of a magneticflux) is y, the y-directional convergence is given by the obliquepassage of charged particles in the inlet and the outlet end magneticfields of the fanlike magnetic field. That is, when the angles formedbetween the vertical plane to the electron beam 30 and the ends of theelectromagnets 31 and 32 have the relations of EP11 and EP12 as shown inFIG. 5A (where the angles EP11 and EP12 have positive signs), theelectron beam 30 is converted in the y direction as shown in FIG. 5B. Onthe other hand, the angles formed between the vertical plane to theelectron beam 30 and the ends of the electromagnets 31 and 32 have therelations of EP21 and EP22 as shown in FIG. 5C (where the angles EP21and EP22 have negative signs), the electron beam 30 is dispersed in they direction as shown in FIG. 5D. By selecting optimal incident andoutgoing angles, the distortion and the aberration of the image may bereduced to a minimum.

In the TRIO, the secondary aberration may be represented as follows.##EQU1## where x is a beam width in the direction of the energydispersion caused by the fanlike magnetic field, α is a spread angle, δis a spread of energy, y is a beam width that is vertical to x, and β isa spread angle in the y-direction. A subscript 1 represents an initialcondition of the beam. A subscript 2 represents a beam width of aconverged image. X, A, D, Y and B represent a first-order aberrationcoefficient. XX, XA, AA and the like represent a secondary aberrationcoefficient.

For the energy filter, it is necessary to meet the following first-orderconvergence condition for keeping an image point in focus with nodistortion caused in the image.

    X=±1, A=0, D=0, Y=±1, B=0                            (3)

At the energy dispersion spot, it is necessary to meet the followingfirst-order convergence condition.

    X=±1, A=0, D≠0                                    (4)

Even if the foregoing conditions are met, the resulting energy filterdoes not offer excellent performance. To keep the distortion of theimage minimum, it is necessary to eliminate the secondary aberration.The secondary aberration coefficients XD, AD, DD, YD and BD about theenergy width δ may be negligible, because the energy width δ₁ of theelectrons are allowed to be restricted to 10⁻⁴ or lower through theslit. Since the other beam spread is as wide as 10⁻³, it is necessary todesign the electron optical system so that the secondary aberrationcoefficient may be made far smaller.

The table 1 lists the values of the aberration coefficients calculatedby the TRIO in the electron optical systems of the omega type energyfilter shown in FIG. 2 and the alpha type energy filter shown in FIGS. 3and 4. The form parameters of each energy filter required for thecalculation are derived from Optik 73, No. 3 (1986) 99-107.

                                      TABLE 1                                     __________________________________________________________________________    Secondary                                                                       Aberration                                                                    Coefficients XX XA AA YY YB BB YX YA BX                                     __________________________________________________________________________    OMEGA type                                                                      (FIG. 2) Image -53 3.7 0.0 -121 15 -0.3 24 -7.3 7.6                            Point                                                                         Dispersion -52 -3.6 -0.2 -28 -4.8 -0.1                                        Point                                                                        ALPHA type                                                                    (FIG. 3) Image -100 6.7 0.0 70 -6.8 0.1 -35 -5.4 12                            Point                                                                         Dispersion -8 -4.8 -0.2 -120 -4.1 -0.2                                        Point                                                                        ALPHA type                                                                    (FIG. 4) Image -46 13 0.0 -5500 1200 -64 -810 -16 110                          Point                                                                         Dispersion -30 -3.8 -0.5 -1500 500 42                                         Point                                                                      __________________________________________________________________________

These values are calculated at an MKS unit. Hence, the aberration is avalue of μm if the beam spread is in the order of 10⁻³. If theaberration at an image point is 100 μm, the image expanded to be 100times larger through the projecting lens system of the electronmicroscope has a distortion of 1 cm, which is not an excellent filteredimage. In particular, the alpha type energy filter such as Perez offersan image point where the secondary aberration is 5 mm, the first-ordermagnification Y in the y direction is 28, and the y-directionalaberration coefficient B is 3. Hence, the resulting image is notconverged and extremely distorted.

The further problems the prior art involves are reduction of magneticpoles in number and compact design, which is due to location of thefilter on the way of a mirror body. The conventional systems rather thanthe alpha type energy filter of Perez, et al. with a large-imagedistortion need four magnetic poles and thus as many end magnetic fieldsfor the incident and outgoing electron beam as eight. The end magneticfields may make the beam convergence uncertain. Preferably, they may besmaller in number.

It is an object of the present invention to provide an electron energyfilter that is designed compactly with a small number of magnetic polesand offers a more excellent convergence characteristic.

To make the incident direction of the electron beam to the filter thesame as the outgoing direction, it is necessary to keep the deflectingangles VM1 and VM2 of the first and the second magnetic fields in thefollowing relation.

    WM1+WM2/2=180°                                      (5)

If the deflecting angle VM1 of the first magnetic field is shiftedlargely from 90°, the area of the magnetic pole is made larger, so thecompact design of the filter is not allowed.

If WM1 and WM2 are substantially 90° and 180°, respectively, theelectron beams in the free space are kept in parallel.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a schematic view showing an energy filter according to anembodiment of the present invention;

FIG. 1B is a section view showing the same as above;

FIG. 2 is a schematic view showing the conventional omega type energyfilter;

FIG. 3 is a schematic view showing the conventional alpha type energyfilter;

FIG. 4 is a schematic view showing another alpha type energy filter;

FIGS. 5A to 5D are explanatory views showing y-directional convergenceof an electron beam caused by the oblique incident and outgoing effect;

FIG. 6 is a graph showing a relation between an incident angle EP11 anda secondary aberration in a first magnetic field;

FIG. 7 is a graph showing a change of a secondary aberration with aratio of a free space distance to a trace radius of the first magneticfield;

FIG. 8 is a graph showing a relation between the incident angle EP21 anythe secondary aberration in the second magnet field;

FIG. 9 is a graph showing a change of a secondary aberration with aratio of a trace radius of the first magnetic field to that of thesecond magnetic field;

FIG. 10 is an explanatory view showing a change of a secondaryaberration with a ratio of a trace radius of the first magnetic field toa convex radius of an incident end;

FIG. 11 is an explanatory view showing a change of a secondaryaberration with a ratio of a track radius of the first magnetic field toa concave radius of an outgoing end;

FIG. 12 is a view showing a construction of an electron microscopeprovided with an energy filter according to the embodiment of thepresent invention;

FIG. 13 is a photograph showing an energy-filtered image formed throughan electron microscope provided with an electron filter according to theembodiment of the present invention;

FIG. 14A is a graph showing an energy dispersing spectrum caused by theenergy filter according to the embodiment of the present invention andFIG. 14B is an expanded view of the C-K core loss peak;

FIGS. 15A to 15C are photographs taken by an electron microscope forshowing the compared images energy-filtered by the energy filteraccording to the embodiment of the present invention; and

FIG. 16 is a schematic view showing an energy filter according toanother embodiment of the present invention.

EMBODIMENTS First Embodiment

FIG. 1 is a schematic view showing the magnetic poles of an electronenergy filter according to an embodiment of the present invention, inwhich FIG. 1A is a plane view and FIG. 1B is a central section. Anelectron beam 4 is deflected through the effect of a first magneticfield generated between a pair of magnetic poles 21 and 21' by virtue ofcurrent flowing through coils 25a and 25b and a second magnetic fieldgenerated between a pair of magnetic poles 22 and 22' by virtue ofcurrent flowing through coils 26a and 26b. After the electron beam 4 isdeflected about 90° by virtue of the first magnetic filed yielded by thefirst pair of magnetic poles 21 and 21', the deflected beam 4 travelsstraight on a first path and enters into the second magnetic fieldyielded by the second pair of magnetic poles 22 and 22', in which thebeam is deflected 180° in the substantially same direction. Then, thebeam travels on the second path in the free space, which issubstantially parallel to the first path, and then is again incident tothe first magnetic field in which the beam is deflected 90° with thesame radius as the first deflection. Hence, the beam is rotated totally360° and then outgoes in the same direction as the incident one. Thetrace radius AM2 of the electron beam in the second magnetic field isabout half as long as the track radius AM1 in the first magnetic field,for reducing the magnetic poles 21 and 22 in area. In the filterconstructed as above, the electron beam is traced as a gamma (γ).

The y-directional convergence effect is brought about by the obliqueoutgoing and re-incident of the beam from and to the magnetic poles 21and 21' for generating the first magnetic field. The deflecting angleWM1 and the central trace radius AM1 in the first magnetic field, thedeflecting angle VM2 and the central trace radius AM2 in the secondmagnetic field, and the distance DL2 from the outgoing point of thefirst magnetic field to the incident point of the second magnetic fieldare selected to keep the following numerical range, for meeting theforegoing expressions (3) and (4).

    80°≦WM1≦100°                   (6)

    2≦AM1/AM2≦3                                  (7)

    160°≦WM2≦200°                  (8)

    0.4AM1≦DL2≦0.8AM1, Preferably 0.5AM1≦DL2≦0.7AM1(9)

More preferably, WM1 is substantially 90°. WM2 is substantially 180°.AM1/AM2 is substantially 2. DL2 is substantially equal to AM2.

The initial incident angle EP11 and the outgoing angle EP12 of theelectron beam in the first magnetic field and the incident and theoutgoing angles EP21 of the electron beam in the second magnetic fieldare selected to keep the following numerical range, for reducing thesecondary aberration to a minimum.

    -12°≦EP1≦0°                    (10)

    30°≦EP12≦40°                   (11)

    -3°≦EP21≦1°                    (12)

In another preferable embodiment, EP11 is substantially 42°, EP12 issubstantially 11.8°, and EP21 is substantially 30°.

The magnetic pole ends of the initial incident point and the outgoingpoint of the electron beam in the first magnetic pole are worked as aconvex and a concave planes in order to reduce the secondary aberrationto a minimum. The curvature radii RM1 and RM2 of these planes areselected to keep the following numerical range, where a sign of +indicates the curvature is convex and a sign of - indicates thecurvature is concave.

Incident Convex Plane:

    0.5≦AM1/RM1≦0.9                              (13)

Outgoing Concave Plane:

    -0.5≦AM1/RM2≦0.05                            (14)

In another preferred embodiment, AM1/RM1 is substantially 1.0 andAM1/RM2 is substantially 0.7.

The representative parameters of the electron optical system used inthis embodiment are as follows.

    EP11=-6°, RM1=0.063 m, AM1=0.05 m,

    WM1=90°, EP12=33.4°, RM2=-0.2 m,

    DL2=0.03 m, EP21=-1.5°, AM2=0.022 m,

    WM2=180°, EP22=EP21=-1.5°                    (15)

Under these parameters, the first-order aberration coefficients at theimage point DL14 and the dispersion point DLD4 are as follows.

    DL14=0.028 m, X=1.0, A=0.0, D=0.0,

    Y=-1.0, B=0.0

    DLD4=0.094 m, X=-1.0, A=0.0, D=0.1,

    Y=-1.6, B=-0.17                                            (16)

These first-order aberration coefficients meet the foregoing expressions(3) and (4).

The secondary aberration coefficients of the electron energy filterconstructed to use these parameters are listed in the following Table 2.

    __________________________________________________________________________    Secondary Aberration                                                            Coefficients of the Invention XX XA AA YY YB BB YX YA BX                    __________________________________________________________________________    Image Points -8.8                                                                             0.6                                                                              0.00                                                                             1.3                                                                              0.3                                                                              0.00                                                                             14 0.1                                                                              0.03                                       Dispersion 1.4 0.8 0.03 -40 -4.3 0.02 -- -- --                                Points                                                                      __________________________________________________________________________

As is obvious from the Table 2, as compared with the conventional omegaand alpha types, the secondary aberration of the image point 7 isreduced down to 1/10 or lower. The secondary aberration at thedispersion point 8 is greatly reduced.

The optima condition of the electron optical system provided in theenergy filter according to this embodiment is determined by varying theparameters. As an example, FIGS. 6 to 11 show the variation of thesecondary aberration brought about by an incident angle EP11 of thefirst magnetic field, a ratio (DL2/AM1) of a free space distance betweenthe first and the second magnetic fields to a trace radius of the firstmagnetic field, a ratio (AM1/RM1) of a trace radius of the firstmagnetic field to a convex radius of an incident end, and a ratio(AM1/RM2) of a trace radius of the first magnetic field to a concaveradius of an outgoing end. The secondary aberration shown in FIGS. 6 to11 are indicated by using the secondary aberration coefficients of theexpressions (1) and (2). IN actual, they are represented in μm on theassumption of the beam spread appearing in using this filter forpractical purpose. The aberration with a bar on it is a value on thedispersion point. The aberration with no bar on it is a value on theimage point.

The optimal form is determined on the result of simulating those values.The allowance for the secondary aberration is not unconditionallydetermined. It depends on the purpose of the device itself. As isapparent from FIGS. 6 to 11, the secondary aberration is graduallyvaried with respect to the abovementoned parameters. It is practicallynegligible if it stays in the range of ±2 μm.

As is understood from FIG. 6, if the incident angle EP11 of the firstmagnetic field is defined in the range of -18° to 5°, the secondaryaberration may be reduced to a value within ±2 μm. If it is defined inthe range of -12° to 0°, it may be further reduced to a value within ±1μm. As is understood from FIG. 7, it is possible to suppress thesecondary aberration within ±2 μm if a ratio (DL2/AM1) of a middle freespace distance to a trace radius of the first magnetic field is set inthe range of 0.4 to 0.8. Further, as is understood from FIG. 8, it ispossible to suppress the secondary aberration within ±2 μm if a ratio(AM1/AM2) of a trace radius of the first magnetic field to a traceradius of the second magnetic field is set in the range of 2 to 3.Moreover, as is understood from FIG. 10, it is possible to suppress thesecondary aberration within ±2 μm if a ratio (AM1/RM1) of the traceradius of the first magnetic field to a convex radius of the incidentend is set in the range of 0.5 to 0.9. As is understood from FIG. 11, itis possible to suppress the secondary aberration within ±2 μm if a ratio(AM1/RM2) of the trace radius of the first magnetic field to the concaveradius of the outgoing end is set in the range of -0.5 to 0.05.

After the electron energy filter of this embodiment is attached with theknown electron microscope (manufactured by Hitach, Ltd., Model: H-8100),the function of the energy filter was checked. FIG. 12 shows theconstruction of the electron microscope attached with the energy filter.In FIG. 12, this electron energy filter has in-column magnetic poles 21and 22 located between a middle lens system 53 and a projecting lenssystem 55. An electron beam 4 is fired by an electron gun 59 and thenconverged through the converging lens system 50. The converted beam 4transmits through a specimen 51 and then is converged to a crossoverpoint 5 through the middle lens system 53. The converted beam goesaround the inside of the energy filter and then converted again at anenergy dispersion point 8. The electrons having respective kinds ofenergy are dispersed through the effect of the magnetic fields of themagnetic poles 21 and 22 so that those electrons having a differentmagnitude of energy are transformed into the corresponding linespectrums. A variable slit 54 is located at the energy dispersion point8 for selectively adjusting the energy width of the line spectrum to aspecific one.

On the other hand, the incident image surface 6 formed through themiddle lens system 53 is formed again on the outlet image surface 7through the effect of the energy filter. This image is not made vague bythe so-called achromatic effect, that is, the effect that thedispersions are offset by the energy filter if the electrons have thecorresponding energy width.

The electron beam selected as the specific energy through the variableslit 54 passes a projecting lens 55, through which an outlet image 7 isexpansively formed on a fluorescent plate 56. A numeral 57 denotes adetector.

The electron energy filter provides two magnetic poles 21 and 22 thatare independent of each other. Each magnetic pole has a coil woundtherearound. The magnetic pole yields a magnetic field for anacceleration voltage of the electrons so that the electron beam travelsalong a specific trace.

FIG. 13 shows an example of a photo representing an energy-filteredimage formed by the electron microscope attached with the energy filteraccording to this embodiment. The used specimen is a carbon grating filmabout 0.5-μm square. The selected energy is electrons with zero loss. Asis obvious from FIG. 13, though the electron beam travels around theinside of the energy filter, the image is represented as a square.

When detecting the electron beam transmitting through the slit under theconstant magnetic field and the increased voltage for accelerating theelectrons, the electron beam is represented as an energy spectrum of thetransmitted electrons, which is illustrated in FIGS. 14A and 14B.

The conventional electron microscope is served to form all the electronshaving any magnitude of energy contained in this energy spectrum as animage and expand it.

The electron energy filter enables to select only the electrons having aspecific magnitude of energy. FIGS. 15A to 15C show electron microscopicphotos of a non-dyeing specimen with a thickness of about 70 μm where acardiac muscle of a mouse is double-fixed by glutaraldehyde and osmiumtetroxide. FIG. 15A shows an ordinary image of the electron microscope.FIG. 15A shows an image formed of only the electrons with zero lossselected through the electron energy filter. FIG. 15C is a loss imageformed of only the electrons close to -250 eV.

As is obvious from the comparison of these photos, the electronmicroscope attached with the energy filter of this embodiment may offeran image with improved contrast rather than the conventional electronmicroscope. Further, in the former, by selecting the core-losselectrons, it is possible to form a vivid mapping image of specificelements and improve a function of an analytic electron microscope.

As set forth above, the resulting electron energy filter may becompactly designed with a small number of magnetic poles and is low inaberration and excellent in a converging characteristic.

Second Embodiment

In order to meet the expressions (3) and (4), the deflecting angle WM1and the central trace radius AM1 in the first magnetic field, thedeflecting angle WM2 and the central trace radius AM2 in the secondmagnetic field, the distance DL2 from the outgoing point of the firstmagnetic field to the incident point of the second magnetic field areselected in the following numerical ranges.

    80°≦WM1≦100°, preferably, substantially 90°                                                (17)

    1.5 AM2≦AM1≦2.5 AM2,                         (18)

    160°≦WM2≦200°, preferably, substantially 180°                                               (19)

    Am1≦DL2≦2 AM1                                (20)

In order to reduce the secondary aberration to a minimum, the firstincident angle EP11 and the outgoing angle EP12 of the electron beam inthe first magnetic field and the incident and outgoing angle EP21 of theelectron beam in the second magnetic field are selected in the followingnumerical ranges.

    20≠≦EP11≦30°                    (21)

    20°≦EP12≦30°                   (22)

    -3°≦EP21≦1°                    (23)

Further, the magnetic pole end of the first incident point and theoutgoing point of the electron beam in the first magnetic field isworked as a convex or a concave. These curvature radii RM1 and RM2 areselected in the following numerical ranges.

    Incident Plane -1≦AM1/RM1≦1                  (24)

    Outgoing Plane -1≦AM1/RM2≦1                  (25)

where a sign of + represents the convex curvature and a sign of -represents the concave curvature.

The energy filter is designed so that the electron beam that outgoes atthe crossover point is, again, focused at one point on the energydispersion surface. This design is based on the following disadvantagesappearing if the electron beam is linearly focused without focusing itat one point,

(1) It is necessary to locate the energy spectrum and the energyselecting slit strictly in parallel to each other. In order to suppressthe adverse effect of the shadow of dust adhering to the slit, it isnecessary to make the slit tabular in a wide range.

(2) It is likely that an imperfection in an axial condition may slip theelectron beam out of the condition of X=Y=1 on the image surface.

(3) To observe a grating pattern, the image surface is formed on theenergy dispersion surface. Since the x-axial focal distance is differentfrom the y-axial focal distance, it is difficult to match the focalpoints of the image surface to each other.

The representative parameters of the electron optical system of thisembodiment are indicated as follows.

    EP11=22.8°, RM1=0 mm, AM1=45 mm,

    WM1=90°, EP12=27.7°, RM2=0 mm,

    DL2=70.4 mm, EP21=0°, AM2=22 mm,

    WM2=180°                                            (26)

In this case, the first-order aberration coefficients at the image pointDLI4 and the energy dispersion point DLD4 are indicated as follows,where DLI4 is a distance from the final magnetic pole end to the imagepoint and DLD4 is a distance from the final magnetic pole end to theenergy dispersion point.

    DL14=30.4 mm, X=1.0, A=0.0, D=0.0,

    Y=-1.0, B=0.0

    DLD4=102.8 mm, X=-1.0, A=0.0, D=0.16,

    Y=1.0, B=0.0                                               (27)

These first-order aberration coefficients meet the expressions (3) and(4).

The secondary aberration coefficients of the electron energy filter ofthis embodiment calculated by the TRIO are illustrated in Table 3. Thosesecondary aberration coefficients are small enough to inhibit thedistorted or vague energy spectrum and image and are maintained in theallowable range. Further, the electron energy filter includes fourdeflecting magnetic fields, that is, the number of the magnetic poles isas small as four. The electron energy filter is excellent in workabilityand constructed compactly.

                                      TABLE 3                                     __________________________________________________________________________    Secondary Aberration Coefficient of                                             Electron Energy Filter of the Invention                                     Aberration                                                                      Coefficient XX XA AA YY YB BB YX YA BX BA                                   __________________________________________________________________________    Image Point                                                                         -43.4                                                                             3.14                                                                             0.00                                                                             -37.9                                                                             2.74                                                                             0.00                                                                             41.1                                                                             -2.41                                                                            -1.48                                                                            0.00                                         Dispersion -105.3 -12.1 -0.44 -138.8 -18.3 -0.67 287.5 18.1 17.9 1.24                                           Point                                     __________________________________________________________________________

Third Embodiment

In order to meet the expressions (3) and (4), the deflecting angle WM1and the central trace radius AM1 in the first magnetic field, thedeflecting angle WM2 and the central trace radius AM2 in the secondmagnetic field, and a distance from the outgoing point of the firstmagnetic field to the incident point of the second magnetic field areselected in the following numerical ranges.

    75°≦WM1≦85°, preferably, substantially 80°                                                (28)

    1.5AM2≦AM1≦2.5AM2                            (29)

    190°≦WM2≦210°, preferably, substantially 200°                                               (30)

    0.8AM1≦DL2≦2.5AM1                            (31)

In order to reduce the secondary aberration to a minimum, the firstincident angle EP11 and the outgoing angle EP12 of the electron beam inthe first magnetic field and the incident and outgoing angle EP21 of theelectron beam in the second magnetic field are selected in the followingnumerical range.

    15°≦EP11≦15°                   (32)

    30°≦EP12≦45°                   (33)

    -7°≦EP21≦3°                    (34)

Further, the magnetic pole end of the first incident point and theoutgoing point of the electron beam in the first magnetic field isworked as a convex or a concave for reducing the secondary aberration toa minimum. These curvature radii RM11 and RM12 are selected in thefollowing numerical range, where a sign of + represents the convexcurvature and a sign of - represents the concave curvature.

    Incident Plane: 0.5≦AM1/RM11≦1.5             (35)

    Outgoing Plane: -1.5≦AM1/RM12≦-0.5           (36)

Likewise, the curvature radius RM21 of the magnetic pole end between theincident point and the outgoing point of the electron beam in the secondmagnetic field is selected in the following numerical range by the traceradius AM2.

    0.5≦AM2/RM21≦1.5                             (37)

This electron energy filter is designed so that the electron beam thatoutgoes at the crossover point is, again, focused at one point on theenergy dispersion surface. If the electron beam is focused not at onepoint but linearly, the following disadvantages take place.

(1) It is necessary to locate the energy spectrum and the energyselecting slit strictly in parallel to each other. Further, to suppressthe adverse effect of the shadow of dust adhering to the slit, it isnecessary to make the slit tabular in a wide range.

(2) It is likely that an imperfection in an axial condition may slip theelectron beam out of the condition of X=Y=1.

(3) To observe the grating pattern, the image surface is formed on theenergy dispersion surface. However, since the x-axial focal distance isdifferent from the y-axial focal distance, it is difficult to match thefocal points of the image surface to each other.

The representative parameters of the electron optical system of thisembodiment are indicated as follows.

    AM1=45 mm, WM1=80°, EP11=1.18°,

    EP12=36.9°, RM11=47.01 mm,

    RM12=-47.52 mm, DL2=73.65 mm,

    AM2=24 mm, WM2=200°, EP21=0°                 (38)

In this case, the first-order aberration coefficients at the image pointDLI4 and the energy dispersion point DLD4 are indicated as follows,where DLI4 is a distance from the final magnetic pole end to the imagepoint and DLD4 is a distance from the final magnetic pole end to theenergy dispersion point.

    DL14=9.83 mm, X=1.0, A=0.0,

    D=0.0, Y=1.0, B=0.0

    DLD4=92.34 mm, X=-1.0, A=0.0,

    D=0.11, Y=1.0, B=0.0                                       (39)

These first-order aberration coefficients meet the expressions (3) and(4).

The secondary aberration coefficients of the electron energy filteraccording to this embodiment, which are calculated by the TRIO, arelisted in Table 4. Those secondary aberration coefficients are smallenough to inhibit the distorted or vague energy spectrum or image andmaintained in the allowable range. This electron energy filter isconstructed to have two deflecting magnetic fields for realizing theforegoing optical system, that is, the number of the magnetic poles isas small as four. The electron energy filter is excellent in workabilityand is constructed compactly.

                                      TABLE 4                                     __________________________________________________________________________    Secondary Aberration Coefficient of                                             Electron Energy Filter of the Invention                                     Aberration                                                                      Coefficient XX XA AA YY YB BB YX YA BX BA                                   __________________________________________________________________________    Image Point                                                                         -43.4                                                                             3.14                                                                              0.00                                                                             -37.9                                                                             2.74                                                                              0.00                                                                             41.1                                                                             -2.41                                                                            -1.48                                                                            0.00                                       Dispersion -105.3 -12.1 -0.44 -138.3 -18.5 -0.67 287.5 18.1 17.9 1.24                                             Point                                   __________________________________________________________________________

Fourth Embodiment

In order to meet the expressions (3) and (4), the deflecting angle WM1and the central trace radius AM1 in the first magnetic field, thedeflecting angle WM2 and the central trace radius AM2 in the secondmagnetic field, and the distance DL2 from the outgoing point of thefirst magnetic field to the incident point of the second magnetic fieldare selected in the following numerical range.

    80°≦WM1≦100°, preferably, substantially 90°                                                (40)

    1.5AM2≦AM1≦2.5AM2                            (41)

    160°≦WM2≦200°, preferably, substantially 180°                                               (42)

    AM1≦DL2≦2AM1                                 (43)

In order to reduce the secondary aberration to a minimum, the firstincident angle EP11 and the outgoing angle EP12 of the electron beam inthe first magnetic field and the incident and outgoing angle EP21 of theelectron beam in the second magnetic field are selected in the followingnumerical ranges.

    38°≦EP11≦45°                   (44)

    8°≦EP12≦16°                    (45)

    25°≦EP21≦35°                   (46)

Further, the magnetic pole end between the first incident point and theoutgoing point of the electron beam in the first magnetic field isworked as a concave or a convex for reducing the secondary aberration toa minimum. These curvature radii RM11 and RM12 are selected in thefollowing numerical ranges, where a sign of + represents a convexcurvature and a sign of - represents a concave curvature.

    Incident Plane -0.2≦AM1/RM11≦0               (47)

    Outgoing Plane 0≦AM1/RM12≦1                  (48)

    Incident and Outgoing Plane 0.5≦AM1/RM21≦1.5 (49)

The electron energy filter is designed so that the electron beam thatoutgoes at the crossover point is, again, focused at one point on theenergy dispersion surface. If the electron beam is focused not at onepoint but linearly, the following disadvantages take place.

(1) It is necessary to locate the energy spectrum and the energyselecting slit strictly in parallel to each other. Further, to suppressthe adverse effect of the shadow of the dust adhering to the slit, it isalso necessary to make the slit tabular in a wide range.

(2) It is likely that an imperfection in an axial condition may slip theelectron beam out of the condition of X=Y=1 on the image surface.

(3) To observe the grating pattern, the image surface is formed on theenergy dispersion surface. However, since the x-axial focal distance isdifferent from the y-axial focal distance, it is difficult to match thefocal points of the image surface to each other.

Next, the representative parameters of the electron optical system ofthis embodiment are indicated as follows.

    EP11=42.1°, AM1=32 mm,

    AM1/RM11=-0.1455, WM1=90°,

    EP12=11.77°, AM1/RM12=0.668,

    DL2=50.92 mm, EP21=30°, AM2=18 mm,

    WM2=180°, AM2/RM21=AM2/RM22=1.0                     (50)

FIG. 16 shows a schematic construction of the filter constructed to usethese parameters, in which the same members of FIG. 16 as those of FIG.1A have the same reference numbers and thus are not descriptive herein.

In this case, the first-order aberration coefficients at the image pointDLI4 and the energy dispersion point DLD4 are indicated as follows,where DLI4 is a distance from the final magnetic pole end to the imagepoint and DLD4 is a distance from the final magnetic pole end to theenergy dispersion point.

    DL14=42.1 mm, X=1.0, A=0.0, D=0.0,

    Y=1.0, B=0.0

    DLD4=111.8 mm, X=-1.0, A=0.0, D=0.2,

    Y=-1.0, B=0.0                                              (51)

These first-order aberration coefficients meet the expressions (3) and(4).

The secondary aberration coefficients of the electron energy filteraccording to this embodiment, which are calculated by the TRIO, arelisted in Table 5. Those secondary aberration coefficients are smallenough to inhibit the distorted or vague energy spectrum and image andthus maintained in an allowable range. Further, the electron energyfilter is constructed to have two deflecting magnetic fields forrealizing the foregoing optical system, that is, the number of themagnetic poles is as small as four. The electron energy filter isexcellent in workability and is constructed compactly.

                                      TABLE 5                                     __________________________________________________________________________    Secondary Aberration Coefficient of                                             Electron Energy Filter of the Invention                                     Aberration                                                                      Coefficient XX XA AA YY YB BB YX YA BX BA                                   __________________________________________________________________________    Image Point                                                                         -2.22                                                                            0.17                                                                             0.00                                                                             -73.2                                                                            5.2                                                                              0.00                                                                             6.18                                                                              1.19                                                                             2.45                                                                             0.08                                          Dispersion -1.06 0.026 0.00 -47.9 0.19 0.00 253 17.7 19.9 1.02                Point                                                                       __________________________________________________________________________

We claim:
 1. An electron energy filter for deflecting electrons invacuum through the effect of magnetic fields, filtering only theelectrons having a specific magnitude of energy, and forming an image ofsaid electrons, comprising:a first pair of magnetic poles for generatinga first deflecting magnetic field and a second pair of magnetic polesfor generating a second deflecting magnetic field directed in the samedirection as said first deflecting magnetic field; and wherein after anelectron beam is incident to said first pair of magnetic poles, saidelectron beam is deflected a first deflecting angle WM1 with a firsttrace radius AM1 through the effect of said first deflecting magneticfield, passed through a free space having a distance DL2 causing nomagnetic field and then is incident to said second deflecting magneticfield, after that, said electron beam is deflected a second deflectingangle WM2 with a second trace radius AM2, passed through said free spacehaving the thickness DL2 causing no magnetic field and again is incidentto said first deflecting magnetic field, after that, said electron beamis deflected said first deflecting angle WM1 with said first traceradius AM1 and then outgoes in the substantially same direction as theincident one of said electron beam to said first deflecting magneticfield, and said first trace radius AM1, said second trace radius AM2,said first deflecting angle WM1, said second deflecting angle WM2, andsaid free space distance DL2 meet the following relation of80°≦WM1≦100°2 AM2≦AM1≦3 AM2 160°≦WM2≦200° 0.5 AM1≦DL2.
 2. An electron energy filteras claimed in claim 1, wherein a first incident angle EP11 and a firstoutgoing angle EP12 of said electron beam against said first deflectingmagnetic field and an incident and outgoing angle EP21 of said electronbeam against said second deflecting magnetic field meet the followingrelation of-12°≦EP11≦0° 30°≦EP12≦40° -3°≦EP21≦1°.
 3. An electron energyfilter as claimed in claim 2, wherein a magnetic pole end at a firstincident point of said electron beam against said first deflectingmagnetic field has a curvature radius of RM1, a magnetic pole end at afirst outgoing point has a curvature radius of RM2, and said curvatureradii RM1 and RM2 meet the following relation of0.5≦AM1/RM1≦0.9-0.5≦AM1/RM2≦0.05.
 4. An electron energy filter as claimed in claim 1,wherein said first incident angle EP11 is about 42°, said first outgoingangle EP12 is about 11.8°, and said incident and outgoing angle EP21against said second deflecting magnetic field is about 30°.
 5. Anelectron energy filter as claimed in claim 4, wherein a magnetic poleend at the incident point of said electron beam against said firstdeflecting magnetic field has a curvature radius of RM1, a magnetic poleend at the outgoing point has a curvature radius of RM2, and aidcurvature radii RM1 and RM2 meet the following relation ofAM1/RM1=1.0AM1/RM2=0.7.
 6. An electron energy filter as claimed in claim 1, whereinsaid first deflecting angle WM1 is about 90°, said second deflectingangle WM2 is about 180°, and the distance DL2 of said free space islonger than about a half of said first trace radius AM1.
 7. An electronenergy filter as claimed in claim 1, wherein said first deflecting angleWM1 is about 90°, said second deflecting angle WM2 is about 180°, andthe distance DL2 of said free space is substantially equal to said firsttrace radius AM1.
 8. A transmission electron microscope including saidelectron energy filter claimed in claim
 1. 9. An electron energy filterfor deflecting electrons in vacuum through the effect of magneticfields, filtering only electrons having a specific magnitude of energy,and forming an image of said electrons, comprising:a first pair ofmagnetic poles (21) for generating a first deflecting magnetic field anda second pair of magnetic poles (22) for generating a second deflectingmagnetic field in the same direction of said first deflecting magneticfield; and wherein incident electrons are deflected a first deflectingangle WM1 with a first trace radius AM1 through the effect of said firstdeflecting magnetic field, passed through a free space having a distanceDL2 causing no magnetic field, and is incident to said second deflectingmagnetic field, after that, said electrons are deflected a seconddeflecting angle WM2 with a second trace radius AM2, passed through saidfree space having the distance DL2 causing no magnetic field and isagain incident to said first deflecting magnetic field, then, saidelectrons are deflected said first deflecting angle WM1 with said firsttrace radius AM1 and outgoes in the substantially same direction as theincident one to said first deflecting magnetic field, and said firsttrace radius AM1, said second trace radius AM2, said first deflectingangle WM1, said second deflecting angle WM2 and the distance DL2 of saidfree space meet the following relation of8°≦ WM1≦100° 1.5 AM2≦AM1≦2.5AM2 160°≦WM2≦200° 2 AM1≦DL2≦AM1.
 10. An electron energy filter asclaimed in claim 9, wherein a first incident angle EP11 and a firstoutgoing angle EP12 of said electron beam against said first deflectingmagnetic field and an incident and outgoing angle EP21 of said electronbeam against said second deflecting magnetic field meet the followingrelation of20°≦EP11≦30° 20°≦EP12≦30° -3°≦EP21≦1°.
 11. An electron energyfilter as claimed in claim 10, wherein a magnetic pole end at a firstincident point of said electron beam against said first deflectingmagnetic field has a curvature radius of RM1, a magnetic pole end at afirst outgoing point of said electron beam against said first deflectingmagnetic field has a curvature radius of RM2, and said curvature radiiRM1 and RM2 meet the following relation of-1≦AM1/RM1≦1 -1≦AM1/RM2≦1. 12.An electron energy filter as claimed in claim 9, wherein said firstdeflecting angle WM1 is about 90°, said second deflecting angle WM2 isabout 180°, and the distance DL2 of said free space is longer than saidfirst trace radius AM1.
 13. An electron energy filter as claimed inclaim 9, wherein a first incident angle EP11 and a first outgoing angleEP12 of said electron beam against said first deflecting magnetic fieldand an incident and outgoing angle EP21 of said electron beam againstsaid second deflecting magnetic field meet the following relationof38°≦EP11≦45° 8°≦EP12≦16° 25°≦EP21≦35°.
 14. An electron energy filteras claimed in claim 13, wherein a magnetic pole end at a first incidentpoint of said electron beam against said first deflecting magnetic fieldhas a curvature radius of RM11, a magnetic pole end at a first outgoingpoint has a curvature radius of RM12, a magnetic pole end at an incidentpoint and an outgoing point of said electron beam against said seconddeflecting magnetic field has a curvature radius of RM21, and saidcurvature radii RM11, RM12 and RM21 meet the following relationof-0.2≦AM1/RM11≦0 0≦AM1/RM12≦1 0.5≦AM1/RM21≦1.5.
 15. A transmissionelectron microscope including said electron energy filter as claimed inclaim
 9. 16. An electron energy filter for deflecting electrons invacuum through the effect of magnetic fields, filtering only theelectrons having a specific magnitude of energy, and forming an image ofsaid electrons, comprising:a first pair of magnetic poles (21) forgenerating a first deflecting magnetic field and a second pair ofmagnetic poles (22) for generating a second deflecting magnetic field inthe same direction as said first deflecting magnetic field; and whereinafter an electron beam is incident to said first pair of magnetic poles(21), said electron beam is deflected a first deflecting angle WM1 witha first trance radius AM1 through the effect of said first deflectingmagnetic field, passed through a free space having a distance DL2causing no magnetic field, and then is incident to said seconddeflecting magnetic field, after that, said electron beam is deflected asecond deflecting angle WM2 with a second trace radius AM2, passedthrough said free space having the distance Dl2 causing no magneticfield, and then is again incident to said first deflecting magneticfield, then, said electron beam is deflected said first deflecting angleWM1 with said first trace radius AM1 and outgoes in the substantiallysame direction as the incident direction of said electron beam to saidfirst deflecting magnetic field, and said first trace radius AM1, saidsecond trace radius AM2, said first deflecting angle WM1, said seconddeflecting angle WM2, and the distance DL2 of said free space meet thefollowing relation of7°≦ WM1≦85° 1.5 AM2≦AM1≦2.5 AM2 190°≦WM2≦210° 0.8AM1≦DL2≦2.5 AM1.
 17. An electron energy filter as claimed in claim 16,wherein a first incident angle EP11 and a first outgoing angle EP12 ofsaid electron beam against said first deflecting magnetic field and anincident and outgoing angle EP21 of said electron beam against saidsecond deflecting magnetic field meet the following relationof-15°≦EP11≦15° 30°≦EP12≦45° -7°≦EP21≦3°.
 18. An electron energy filteras claimed in claim 17, wherein a magnetic pole end at a first incidentpoint of said electron beam against said first deflecting magnetic fieldhas a curvature radius of RM11, a magnetic pole end at a first outgoingpoint of said electron beam against said first deflecting magnetic fieldhas a curvature radius of RM12, a magnetic pole end at an incident andoutgoing point of said electron beam against said second deflectingmagnetic field has a curvature radius of Rm21, said curvature radiiRM11, RM12 and RM21 meet the following relation of0.5≦AM1/RM11≦1.5-1.5≦AM1/RM12≦-0.5 0.5≦AM1/RM21≦1.5.
 19. An electron energy filter asclaimed in claim 16, wherein said first deflecting angle WM1 is about80°, said second deflecting angle WM2 is about 200°, and the distanceDL2 of said free space is longer than said first trace radius AM1.
 20. Atransmission electron microscope including said electron energy filteras claimed in claim
 16. 21. An electron energy filter for deflectingelectrons and filtering only the electrons having a specific magnitudeof energy, comprising:means for generating a first deflecting magneticfield, means for generating a second deflecting magnetic field, and afree space on which said magnetic fields of said means have nosubstantial effect; and wherein incident electrons are deflected a firstdeflecting angle WM1 with a first trace radius AM1 through the effect ofsaid first deflecting magnetic field, passed through a first path ofsaid free space, and then are incident to said second deflectingmagnetic field, after that, said electrons are deflected a seconddeflecting angle WM2 with a second trace radius AM2, passed through asecond path of said free space, said second path being substantially inparallel to said first path, and are incident to said first deflectingmagnetic field, then, said electrons are deflected said first deflectingangle WM1 with said first trace radius AM1 and outgo in thesubstantially same direction as the incident direction of said electronsto said first deflecting magnetic field, said outgo electrons due tosaid first deflection magnetic field being deflected to thesubstantially same direction as the incident direction of said electronsincident onto said first deflecting magnetic field.
 22. An electronenergy filter as claimed in claim 21, wherein said first path issubstantially equal in distance to said second path and said distanceDL2 is longer than about a half of said first trace radius AM1.
 23. Anelectron energy filter as claimed in claim 21, wherein said first pathis substantially equal in distance to said second distance and saiddistance Dl2 and said first trace radius AM1 meet the following relationofAM1≦DL2≦2 AM.
 24. A transmission electron microscope including saidelectron energy filter as claimed in claim
 21. 25. A method forfiltering energy of an electron beam, comprising the steps of:deflectingincident electrons a first deflecting angle WM1 with a first traceradius AM1 and passing a first path of a free space causing nosubstantial magnetic field through an effect of a first deflectionmagnetic field; deflecting said electrons passed through said first patha second deflecting angle WM2 with a second trace radius AM2 through aneffect of a second deflection magnetic field and letting said deflectedelectrons pass through a second path of said free space, said secondpath being substantially in parallel to said first path; deflecting saidelectrons passed through said second path said first deflecting anglewith said first trace radius AM1 through the effect of said firstdeflection magnetic field and letting said deflected electrons to outgoin the substantially same direction as said incident electrons; anddeflecting said outgo electrons due to said first deflection magneticfield to the substantially same direction as the incident direction ofsaid incident electrons incident onto said first deflection magneticfield.